Of the electrically programmable and erasable nonvolatile memories, a flash memory is known as the bulk erasable one. The flash memory is excellent in portability and impact resistance, and can be electrically erased in bulk. Therefore, its demand as a memory device for small portable information devices has been rapidly expanding in recent years. Reduction of bit cost, increase of operation speed, and improvement in reliability have been required for the flash memories so that high density data can be safely handled at a high speed.
In some flash memories such as NOR-type and AND-type flash memories, reduction in bit cost is achieved by the technologies for the device structures and the scaling down and the adoption of the multi-bit storage, and further, high-speed programming is achieved by a channel hot electron injection method.
For example, in Japanese Patent Application Laid-Open Publication No. 2004-152977 (Patent Document 1), a technology for achieving both the reduction in bit cost and the high-speed programming is described. In this case, small-area memory cells are realized by forming the bit lines in a memory cell from inversion layers, and high-speed programming is realized by adopting a source-side channel hot electron injection method.